(KRON) -- A convicted child molester was found dead inside his cell on Tuesday at San Quentin state prison in Marin County. Inmate Mark Squires' death is being investigated as a homicide, California Department of Corrections and Rehabilitation officials said. Squires' cellmate has since been placed in restricted housing at the prison, also known as San Quentin Rehabilitation Center. Squires, 70, was serving a life sentence with possibility for parole of sexually abusing a child who was less than 14 years old in Riverside County. The inmate had been locked up in San Quentin for the past 24 years. Prison mugshots of Mark Squires and Gustavo Lopez were released by CDCR. At 3:30 p.m. Tuesday, prison staff members responded to an alarm and approached a cell occupied by Squires and his cellmate, 36-year-old Gustavo Lopez. Squires was "unresponsive" in his cell, and he was declared deceased less than an 30 minutes later. A condemned inmate is led out of his east block cell in San Quentin State Prison, in San Quentin, Calif. (AP Photo / Eric Risberg, File) "Lopez was placed in restricted housing pending investigation by SQRC InvestigativeServices Unit and the Marin County District Attorney’s Office. The Office of theInspector General was notified, and the Marin County Coroner will determine Squires’official cause of death," CDCR officials wrote. Ex-rocker asked Palo Alto woman for help with murder plot, witness testifies Lopez has been incarcerated at San Quentin for four years and is serving a 13 year sentence. He was also convicted of crimes in Riverside County, including kidnapping and false imprisonment with violence.